The present invention is directed to a closure for an open-ended medical implant that receives a rod or related rod-like structure in an open end of the implant and then the rod is captured within the end of the implant by the closure.
A wide range of medical implants are utilized within the human body. With respect to the spine these implants include bone screws that are implanted in vertebrae and various types of connectors and the like that are used to interconnect with bone screws and each other to form an overall support matrix or spinal implant system. Many of these implants, especially bone screws, but also hooks and connectors, are open-ended having a yoke with a pair of upstanding arms that receive rods or other structures in a channel formed between the arms. Typically, each yoke receives an elongate rod, but other elements of the system having cylindrical shaped nipples or the like may also be received in the yokes. Because such bone screws and the like are open-ended, some closure must be used in order to capture the rod or other structure received within the open end of the implant.
To insure that the closure remains secured to and on the bone screw or other implant and that the rod or other structure being held in the implant does not slip either axially or rotationally, in many systems a substantial amount of torque must be applied in installing the closure to both maintain the closure in position and urge the closure against the rod so as to lock the rod in position relative to the bone screw or other implant. The force required to torque the closure presents either a destabilizing force to the overall completed implant in that it may spread the arms or alternatively, if the closure has been made in such a way to counteract the destabilizing force of the torque, the closure and overall implant normally are too large to be used effectively within the system or the closure becomes very complex and difficult to manufacture and/or use.
In particular, the prior art has developed plug closures which are simply threaded cylinders that are screwed into threads on the inside of the arms in the open end of the implant to hold the rod or the like in place. High torque exerted on the plug tends to spread the arms during installation or the arms of the implant may later spread due to forces acting on the implant system, thereby loosening the closure and possibly allowing the rod to slip or the plug to work loose from the bone screw.
In addition, the rods are normally curved or non-linear at the location where they are placed in the implant. That is, typically, the spine is not straight and rods used for supporting the spine are bent to follow the curvature of the spine. Consequently, the rod often has a substantial amount of curvature where the closure plugs engage the rod. Because of this curvature, the rod often is somewhat arcuate underneath the plug even after the plug is fully installed. This presents substantial problems, if the rod is later subjected to forces in use in the body, such as where the patient encounters a fall or automobile accident, that cause the rod to bow somewhat against the desired curvature. This then spaces the rod from engagement with the plug or at least reduces the frictional engagement of the plug with the rod to a level that allows the rod to easily rotate or move axially and/or the plug to become loose and even work free of the bone screw over time.
Other types of prior art closure caps are not threadably received as a plug by the implant, but rather have a complex system of interlocking parts with the implant in order to bridge the gap between the arms. Such caps normally slide into place and then are locked by tightening a set screw or the like. Such closures are very difficult to manufacture and are comparatively very expensive. Closures of the slide-in type, in general, are often also difficult to install, because the parts are small and the working space provided to the surgeon during surgery is very limited. Further, the curvature of the rod may make it very difficult to slide the cap into place so that it may then interlock with mating elements in the bone screw.
Yet another type of closure of the prior art has been a closure that provides a ring that goes entirely about the outside of the arms of the implant. Such a ring may be threaded and received on mating threads on the outsides of the arms of the implant. The ring concept works well in preventing the arms of the implant from spreading due to forces. However, this type of system is subject to problems. One of the problems is that like the previous noted system, when the ring secures a curved rod, the rod may later flex so as to release pressure and reduce friction between the closure and rod, so that the rod becomes loose within the implant and can slip by rotation or axial movement therein. Such systems using a plug are usually not very effective in locking the rods in place, equally as the plug normally engages part of the ring and not the rod, so that direct friction of the plug on the rod does not occur.
Furthermore, the outside circumferential ring substantially increases the thickness of the implant along the axis of the rod. This presents a substantial problem, since there is often very limited space along the rod for all of the various connectors, bone screws and the like that are necessary for the overall implant system and such large closures take up too much space. Additionally, the large caps that are necessary for closing this type of implant leave very little room for the surgeons to work and block access of bender devices to the rod that are needed to shape the rod in some types of implant surgery which require that the curvature of the rod to be modified after connection to the bone screws. The large head of such caps is also contrary to the general desire to reduce size, weight and overall volume of such implants so as to reduce their physical impact on the patient.
It is noted that other problems with the prior art include that, while some types of caps prevent outward movement of the arms, such allow the tops of the arms to bend radially inward toward each other so the cap becomes loose. Further, where the cap is threaded and goes over the outside of the arms, it is often very difficult to line up the threads properly between opposite arms and the cap because of the tight working space, so the cap does not sit correctly and does not tighten correctly against the rod.
A closure cap for an open-ended implant wherein the implant has a pair of upstanding arms located on either side of a rod receiving channel and wherein the arms are interiorly or inwardly threaded to threadably receive the closure. The closure is an incomplete round cap having opposed and equally sized segments or sectors missing so that the cap does not form a complete circle. When positioned to act as a closure, the missing segments allow the closure cap to be comparatively thin in profile relative to a cap of the same radius that is complete. The cap includes an upper plate joined to opposed flanges. The flanges are snugly received around the arms of the implant in use.
The closure includes a central cylindrical member or boss that depends from the plate and is positioned between but in spaced relation to the flanges. The cylindrical boss is sized and shaped to maintain the spacing of the arms when installed on the implant by keeping the arms from bending inwardly. The boss is outwardly threaded to mate with the threads on the interior of the implant arms, so that the boss may be screwed in place between the arms and so that when the boss is fully screwed into the implant head, the flanges are positioned radially outward and in close relationship to the arms to prevent the arms from spreading outwardly.
The plate and the cylindrical boss also include a central top to bottom threaded bore which receives a set screw. In use the set screw engages and is urged under torque against a rod within the channel so as to fix in position and lock the rod against movement relative to the implant.
A set of tools is also provided for use with the invention. A cap installation tool is temporarily threadedly received in the threaded bore to allow the grasping and turning of the closure cap during installation. The installation tool is removed, when the cap is positioned such that the threads of the boss substantially engage and are threaded into the threads of the arms. Once the installation tool is removed and the surgeon is satisfied with the position of the rod relative to the implant, an anti-torque tool is placed over the cap to hold the cap in position and a set screw is positioned within the cap bore. A set screw installation tool is then utilized to torque the set screw which may be a break-off head type wherein the head breaks away at a preselected torque or a non-break-off head type set screw. Once the set screw has been properly torqued, the anti-torque tool and set screw installation tool are removed and the implant is thereafter complete.
A removal tool is also provided which comprises a T-shaped handle with a lower receptacle that receives the cap and allows the cap to be rotated. The cap in this manner can be rotated counterclockwise and removed, should it is necessary to do so for some reason after installation.
Therefore, the objects of the present invention are: to provide a closure for an open-ended medical implant that is comparatively thin in profile along an axis of a rod or other structure received in the implant, that resists movement of the upper ends of the implant arms either radially inward or outward subsequent to installation of the cap and that provides a set screw that secures the rod within the implant at the time of installation and at subsequent times when the normal curvature of the rod is bowed during use; to provide such a closure which comprises an upper plate joining a pair of depending flanges that are received around and in touching or abutting relation to the arms of the implant; to provide such a closure wherein the flanges are opposed and together extend approximately 180xc2x0 about the implant; to provide such a closure having a central depending boss that is threaded to mate with threads on the interior sides of the arms; to provide such a closure in combination with the implant and with a set screw that is highly effective in locking the rod or the like in position relative to the implant and preventing subsequent movement of a rod or other structure either rotationally or axially relative to the implant; to provide such a closure utilizing a relatively small diameter set screw to secure the rod that is much less subject to loosening in comparison to the closure should the rod flex; to provide such a closure which is relatively easy to manufacture and inexpensive to produce; to provide such a closure which is relatively easy to install and may be installed with the rod substantially bent within a rod receiving channel of the implant; to provide such a closure which is relatively easy to remove, should removal be necessary; to provide a set of tools for installing such a closure including an installing tool to facilitate threading of the closure onto the implant; to provide such a set of tools including an anti-torque tool and a set screw installing tool to function in combination to install a set screw in the closure; and to provide a removal tool that captures the closure and allows rotation of the closure relative to the implant; and to provide an overall closure, implant system and set of tools that are especially easy to use and well adapted for the intended purpose thereof.
Other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein are set forth, by way of illustration and example, certain embodiments of this invention.
The drawings constitute a part of this specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.